1. Field of the Invention
The present invention relates to optical communication such as visible light communication, and more particularly to a method and an apparatus for performing bi-directional communication by using a retro-reflector.
2. Description of the Related Art
With the improvement in light-emitting efficiency and the drop in cost of Light Emitting Diodes (LEDs), the usage of LEDs has spread not only in the market of special lighting, such as portable devices, displays, automobiles, traffic lights, billboards, etc. but also in the market of general lighting, such as fluorescent lamps, glow lamps, etc. The light emitting efficiency of white LEDs has already exceeded that of fluorescent lamps, and are now appearing on the market. Further, due to the depletion of Radio Frequency (RF) band frequencies, the possibility of entanglement between various wireless communication technologies, the increase in the requirement for the security of communication, the advent of ultra-high speed ubiquitous communication environment using 4th Generation (4G) wireless technology, etc., attention paid to the optical wireless technology, which is complementary to the RF technology, is being and research into visible light wireless communication using a visible light LED is being conducted by various companies and organizations.
Portable mobile apparatuses, such as mobile phones or Personal Digital Assistants (PDA), and compact digital products, such as a digital camera or a Motion Picture Experts Group (MPEG) audio layer-3 (MP3) player, are equipped with an Infrared Data Association (IrDA) module based on infrared rays, and extensive research on peripheral interfaces for inter-apparatus communication has already been conducted and relevant products thereof have been developed and commercialized. Differently from RF communication, such as Bluetooth™ or Zigbee™, the infrared ray wireless communication is advantageous in that it allows for non-entanglement between apparatuses, has good security, and can be implemented with low power.
However, in the case of existing infrared ray communications, since the communication link is invisible to user's eyes, it is necessary to periodically output infrared ray signals so as to scan surrounding apparatuses. Therefore, the inevitable output of infrared ray signals, even in the case of not performing communication, causes wasteful resource consumption. Further, in the case of existing infrared ray communications, since the infrared rays are invisible to user's eyes, the infrared ray signals are widely radiated with a divergence angle of 30 degrees and the communication link is aligned through adjustment of each apparatus by users. Therefore, the wide divergence angle serves as another reason of wasteful resource consumption.
In the case of optical communications such as visible ray communication, the characteristics of light allow only single directional transmission, and bidirectional communication is possible when a transmitting unit and a receiving unit of the two apparatuses communicating with each other are aligned to each other. In aligning the communication link, the users identify the visible ray signals. At this time, when both an exact alignment of a communication direction from a first apparatus to a second apparatus and an exact alignment of a communication direction from the second apparatus to the first apparatus have been achieved, bidirectional communication is possible. However, such separate alignment of each communication link for both directions is an inconvenience to users for communication. Further, when either of the apparatuses is moved, the alignment of communication directions becomes more difficult, so as to disturb smooth signal transmission. As a result, there is a need for a method of setting a communication link, which is more efficient than the existing methods.